1. Field of the Invention
The present invention relates to a two-dimensional solid-state imaging device, and in particular, to a two-dimensional solid-state imaging device which detects near-field light generated by surface plasmon polariton resonance excited by incoming electromagnetic waves.
2. Description of the Related Art
A two-dimensional solid-state imaging device, such as a digital still camera, a video camera, or a camcorder (an abbreviation of a camera-and-recorder having a photographing section called a video camera and a recording section as a single body), is increasingly provided in which a subject is photographed by a photoelectric conversion element including a two-dimensional solid-state imaging element to form an image. With regard to the pixel size of a CCD imaging element or a CMOS imaging element as a mainstream of a solid-state imaging element, with advancement of miniaturization, the physical size of one pixel is about two times larger than the wavelength of visible light. In the photoelectric conversion element, if the incoming area of an incoming electromagnetic wave to the photoelectric conversion element is small, that is, if the opening area of the photoelectric conversion element with respect to the incoming electromagnetic wave is small, the sensitivity of the photoelectric conversion element is lowered as a whole. There are various measures, such as improvement of condensing efficiency by an on-chip lens (for example, see JP-A-9-148549), securement of an opening area by transistor sharing (for example, see Takahashi Hidekazu, The Journal of the Institute of Image Information and Television Engineers, Vol. 60, No. 3, p. 295-298, 2006), or reduction of optical loss in a waveguide according to the use of a rear surface irradiation structure (see Japanese Patent No. 4124190). According to these methods, however, there is no suggestion of an ultimate solution for the problem in that “if the effective opening area of the photoelectric conversion element with respect to the incoming electromagnetic wave is small, the sensitivity of the photoelectric conversion element in each pixel is lowered”.
Meanwhile, if light is irradiated onto the metal surface having a specific periodic pattern, a resonance mode [surface plasmon polariton (SPP)] is carried out at the metal surface due to an electromagnetic wave of a frequency component corresponding to the periodic pattern, and an electric field (near-field light) which is 102 to 103 times stronger than normal light is generated due to plasmon resonance (for example, see L. Salomon et al., Physical Review Letters, Vol. 86, No. 6, p. 1110-1113, 2001, and “Fundamentals of Optical Technologies”, Masuo Fukui and Motoichi Ohtsu (Ohmsha), ISBN 4-274-19713-1).
Near-field light is non-propagating light caused by the closed lines of electric force generated in a region away from the surface of a substance only by a small distance as compared with the wavelength of an electromagnetic wave when the electromagnetic wave (light) is irradiated onto the substance. In a region away from the substance by a distance equal to or greater than the wavelength of the incoming electromagnetic wave, near-field light is attenuated exponentially and may not be observed. Accordingly, only when a photoelectric conversion element is arranged close to a region where surface plasmon polariton is excited, near-field light can be detected as an electrical signal. Near-field light can be condensed on a spot smaller than the diffraction barrier with sensitivity to the structure smaller than the diffraction barrier uniquely determined by the wavelength of the incoming electromagnetic wave. Then, the electric field intensity is stronger near the generation point of near-field light, and stronger than propagating light by two digits to three digits in a region close to the generation point by about 10 nm. For this reason, even when the opening area of the photoelectric conversion element with respect to the incoming electromagnetic wave is small, the electric field intensity can be increased. Thus, if the photoelectric conversion element is close to the generation point, such that “the effective opening area of the photoelectric conversion element is small”, an ultimate solution is obtained against the problem in the related art in that “the sensitivity of the photoelectric conversion element in each pixel is lowered”.
For example, a photodetection element using near-field light is described in JP-A-2008-233049. According to the technique described in JP-A-2008-233049, a minute resonator 301 is formed on a substrate 302 made of Si with an insulating film 303 formed of SiO2 to have a thickness of 5 nm interposed therebetween. The minute resonator 301 and the insulating film 303 include nanodots having a planar shape of a scalene triangle. Light incident on the minute resonator 301 causes localized surface plasmon resonance, such that an increase in the electric field occurs at three apexes of the metal nanodots, thereby generating strong near-field light. Photodiode regions 304 are formed near the apexes of the metal nanodots, and independently receive the light intensity near the three apexes. The photodiode regions 304 output the electric field intensity as electric charges, and the electric charges are amplified by an amplification transistor 305 after being accumulated and output by a row selection transistor 306 with positional information.